Makefile: fix make build failure when environment of GOPATH have multiple items (#5262)

* fixed make build failure when enviroment of GOPATH have Multiple items

gogs source in /other/path/to/go/src/github.com/gogs/gogs and then

>cd /other/path/go/go/src/github.com/gogs/gogs
>GOPATH=/path/to/go:/other/path/to/go make

will also build success in this fixed code

* in makefile rename gogits to gogs

Makefile

@@ -1,5 +1,5 @@
-LDFLAGS += -X "github.com/gogits/gogs/pkg/setting.BuildTime=$(shell date -u '+%Y-%m-%d %I:%M:%S %Z')"
+LDFLAGS += -X "github.com/gogs/gogs/pkg/setting.BuildTime=$(shell date -u '+%Y-%m-%d %I:%M:%S %Z')"
-LDFLAGS += -X "github.com/gogits/gogs/pkg/setting.BuildGitHash=$(shell git rev-parse HEAD)"
+LDFLAGS += -X "github.com/gogs/gogs/pkg/setting.BuildGitHash=$(shell git rev-parse HEAD)"
 
 DATA_FILES := $(shell find conf | sed 's/ /\\ /g')
 LESS_FILES := $(wildcard public/less/gogs.less public/less/_*.less)
@@ -14,6 +14,7 @@ RELEASE_ROOT = "release"
 RELEASE_GOGS = "release/gogs"
 NOW = $(shell date -u '+%Y%m%d%I%M%S')
 GOVET = go tool vet -composites=false -methods=false -structtags=false
+GOPATH = $(shell echo $${PWD%%src*})
 
 .PHONY: build pack release bindata clean